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Two sexes & sexual Selection

Overview of why female/male system selected for and sexual selection

Jennifer Dever

on 12 March 2015

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Transcript of Two sexes & sexual Selection

Anisogamy Leads to Sexual Dimorphism & Sexual Selection
 Different gamete size requires different organ architecture
 Different gamete size requires different behavioral strategies

Sexual Selection augments Sexually dimorphic traits
certain traits that do not contribute directly to an individual’s survival (or that of their offspring) may be involved with mate acquisition/choice, and therefore are under a different selection pressure (sexual selection).
Females are the "choosier" sex
Females – spend energy on egg production ability (yolk – expensive)
Favor high quality mates
Fewer chances to produce successful offspring
Females choose by:
Resources controlled by males
Traits of males

Males more affected by sexual selection than females
Males – (sperm is cheap) spend energy on mate selection ability, therefore under greater sexual selection pressure

Magnitude of sperm production favors fertilization of eggs from many females
Therefore traits that increase their ability to acquire many mates affected, enhance their competitive ability

Why two distinct Sexes?
Anisogamous gametes selected for over isogamous gametes:
Traits under sexual selection
Fighting ability
Calling ability
Body shape/size
Nuptial gifts

Two sexes & sexual Selection
Ciliates - are differentiated into mating types, a kind of self-not-self discrimination system, as opposed to true sexes
Ciliates - are differentiated into mating types, a kind of self-not-self discrimination system, as opposed to true sexes
(new mutant can easily mate with several other types)
Algae & Fungi have isogomous gametes
When a range of gamete sizes are produced in a population, individuals producing gametes near either the small or large ends of the size distribution will be more successful than individuals who produce medium sized gametes.
Individuals that produce few, but large gametes provide large supply of nutrients and thus they will have larger, healthier offspring at birth.
Individuals that produce many, small gametes can potentially parent more offspring – they can distribute abundant gametes more widely… Small gametes more likely than larger gametes to win the race to fertilization.
The two strategies coexist in populations because each strategy individually is unstable. The most stable situation is a population where 50% are large, 50% small.
Female = the form of the organism that carries the most provisioning for the offspring (i.e., the egg). All other "sexes" are by definition males.

Defined as: directional selection that acts on genetically variable phenotypic traits that affect the reproductive success of individuals of a particular sex.
Sexual Dimorphism = phenotypic difference between males & females
Ptilonorhynchus violaceus

Why do females prefer certain phenotypic traits?
Ciliates are the exception
Is having more than two sexes possible?
Full transcript